Homework Help: Electric Fields In Matter

1. May 26, 2008

qspeechc

1. The problem statement, all variables and given/known data
A thick, spherical shell (inner radius a, outer radius b) is made of dielectric material with a 'frozen in' polarization
$$\textbf{P} = \frac{k}{r} \textbf{\hat r}$$
where k is a constant and r the distance from the centre. (There is no free charge). Find the electric field in each o the three regions by two methods:

a) Locate all the bound charge, nd use Gauss' Law to calculate the field it produces
b) Use $$\oint \textbf{D}.d\textbf{a} = Q_{f enc}$$ to find $$\textbf{D}$$ and then find the electric field

3. The attempt at a solution

a) bound charge density is $$\rho _b -\nabla \textbf{P} = -frac{k}{r^2}$$

Gauss' Law: $$\oint E.da = Q_{enc}/\epsilon _0$$
In the dielectric material:
$$Q_{enc} = \int \rho _b = -4\pi \int _a^r dr '= 4\pi k(a-r)$$
I integrated from a to r because that's the only region with charge.
From Guass' Law:

$$E4\pi r^2 = 4\pi k(a-r)/\epsilon_0$$

$$E = k(a-r)/(\epsilon_0 r^2)$$

For r<a the electric field is zero because there is no bound charge. Similarly, outside the sphere
$$E = k(a-b)/(\epsilon_0 r^2)$$

b)
$$\oint \textbf{D}.d\textbf{a} = Q_{f enc} = 0$$
because there is no free charge. So
$$\textbf{D} = \epsilon _0 \textbf{E} + \textbf{P} = 0$$

$$\textbf{E} = -\textbf{P}/\epsilon_0$$

Where did I go wrong here?